PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE The role of magmatically driven lithospheric thickening on arc 10.1002/2014GC005355 front migration Key Points: L. Karlstrom1, C.-T. A. Lee2, and M. Manga3 Arc front migration occurs globally in continental and some oceanic 1Department of Geophysics, Stanford University, Stanford, California, USA, 2Department of Earth Science, MS-126 Rice settings University, Houston, Texas, USA, 3Department of Earth and Planetary Science, University of California at Berkeley, Berkeley, Crustal thickening causes arc front California, USA migration and truncates mantle melting Tectonic extension may balance crustal thickening for stationary arc Abstract Volcanic activity at convergent plate margins is localized along lineaments of active volcanoes fronts that focus rising magma generated within the mantle below. In many arcs worldwide, particularly continen- tal arcs, the volcanic front migrates away from the interface of subduction (the trench) over millions of Correspondence to: years, reflecting coevolving surface forcing, tectonics, crustal magma transport, and mantle flow. Here we L. Karlstrom, [email protected] show that extraction of melt from arc mantle and subsequent magmatic thickening of overlying crust and lithosphere can drive volcanic front migration. These processes are consistent with geochemical trends, Citation: such as increasing La/Yb, which show that increasing depths of differentiation correlate with arc front Karlstrom, L., C.-T. A. Lee, and migration in continental arcs. Such thickening truncates the underlying mantle flow field, squeezing hot M. Manga (2014), The role of mantle wedge and the melting focus away from the trench while progressively decreasing the volume of magmatically driven lithospheric thickening on arc front migration, melt generated. However, if magmatic thickening is balanced by tectonic extension in the upper plate, a Geochem. Geophys. Geosyst., 15, steady crustal thickness is achieved that results in a more stationary arc front with long-lived mantle melt- doi:10.1002/2014GC005355. ing. This appears to be the case for some island arcs. Thus, in combination with tectonic modulation of crustal thickness, magmatic thickening provides a self consistent model for volcanic arc front migration and Received 24 MAR 2014 the composition of arc magmas. Accepted 30 MAY 2014 Accepted article online 5 JUN 2014 1. Introduction One of the most distinctive geographic features on Earth is the series of long arcuate chains of volcanoes on the upper plate of subduction zones, where cold and hydrothermally altered oceanic lithosphere descends into the Earth’s deep interior. Arc volcanism forms one of the primary connections between long- term climate, landscape, and mantle dynamics. For example, convergent margin igneous activity is one of the main drivers of crustal differentiation and the formation of continents. Arc volcanoes create mountain ranges high and long enough to influence large-scale atmospheric circulation. They are also a significant source of volatiles, such as H2O, CO2, and SO2, to the atmosphere and hydrosphere. Surface topography, composition of erupted magmas, and the volume of volatiles released all depend on the nature of magma generation in the mantle and how these magmas interact with the upper plate. Active volcanism in subduction zones is spatially focused into a narrow, 10–30 km wide, lineament called the arc front, which varies in distance from several tens to several hundreds of kilometers from the trench. The dis- tance of the arc front from the trench must be a manifestation of the thermal state of the mantle wedge or subducting slab and is thus of particular interest. One view is that arc magmatism is driven by hydrous flux melting of the mantle wedge, the fluids being derived from dehydration of the subducting slab as it under- goes prograde metamorphism over a narrow temperature interval [Grove et al., 2009]. If arc magmatism is limited by dehydration reactions in the slab, the position of the arc front may depend primarily on the diffu- sive time scale for slab heating. This time scale sets the temperature structure in the overlying mantle wedge and determines whether rising melts freeze or ascend into the crust. Another view is that arc mag- mas originate from the hot nose of the mantle wedge, melting via decompression that is independent of dehydration reactions in the slab [England and Katz, 2010]. Both scenarios predict some dependency of vol- cano location on subduction parameters slab velocity V and dip angle d that control flow in the wedge, and indeed, a number of studies have shown that the positions of modern arc fronts correlate negatively with slab dip or some product of slab dip and plate velocity [Syracuse and Abers, 2006; Grove et al., 2009; England and Katz, 2010]. KARLSTROM ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 1 Geochemistry, Geophysics, Geosystems 10.1002/2014GC005355 a. 150 100 50 Relative distance Relative (km) trench to 0 b. 0.71 0.708 Sr/ Sr 0.706 87 86 0.704 c. 80 Sierra Nevada 60 Peninsular Ranges Andes 0-25 Ma Andes 25-69 Ma 40 Andes 69-120 Ma La/Yb Lesser Antilles Marianas 20 0 020406080100120 Time (Ma) Figure 1. (a) Crystallization ages of volcanic and plutonic rocks and relative distances to the trench for three continental (Andes, Sierra Nevada, Penninsular Ranges) and two oceanic arcs (Lesser Antilles, Marianas). Andean volcanic data have been divided into three episodes and detrended to focus on the cycles of volcanic migration. Corresponding geochemical indices through time for continental arcs covary with spatial migration: (b) Initial 87Sr/86Sr isotopic ratio and (c) ratio of trace elements La/Yb. Independent of the mechanism of melting, a longstanding extension of this correlation between subduc- tion parameters and arc front location has been that changes in slab dip with time will move the front posi- tion relative to the trench [Dickinson and Snyder, 1978]. For example, the eastward migration of arc magmatism in the Sierra Nevada Batholith, California during the Late Cretaceous is widely attributed to pro- gressive shallowing of the angle of eastward subduction of the Farallon oceanic plate beneath North Amer- ica [Coney and Reynolds, 1977; Dickinson and Snyder, 1978; Lipman, 1992; Humphreys et al., 2003]. Similar arguments have been used to explain the migration of arc fronts in Tibet [Chung et al., 2005], Southeast China [Li and Li, 2007], and the Andes [Haschke et al., 2002]. It has also been argued that mechanical erosion by the downgoing plate may drive the migration of arc fronts [Scholl and von Huene, 2007]. Any successful model of subduction zones, however, must satisfy some key observations related to arc front migration. Some arcs migrate, some do not, and in those that do, migration is not always continuous (Figure 1a). Continental arc volcanism generally migrates away from the trench [Dickinson and Snyder, 1978], some- times in cycles of spatial advance and retreat of volcanic activity with intervening temporal gaps in magma- tism [Haschke et al., 2002]. Some oceanic arc fronts remain stationary relative to the trench or migrate without temporal gaps in eruptive output [Stern et al., 2003]. These differences appear to correspond to var- iations in the overall tectonic state of the overriding plate: oceanic arcs (e.g., Mariana, Tonga) are often strongly extensional, to the point of back-arc basin seafloor spreading, while some continental arcs (e.g., Andes) evolve in the presence of tectonic shortening and subduction erosion of the accretionary wedge [Uyeda and Kanamori, 1979; von Huene and Scholl, 1991]. Spatial migration of arcs also involves changes in the nature of magma transport, differentiation, and inter- action with the upper plate as evidenced by evolving geochemistry as the arc front migrates. For example, the isotopic ratio 87Sr/86Sr (Figure 1b) and bulk silica content increase as continental arcs migrate away from the trench, suggesting longer magma transport times and crustal storage (we note that it is the combi- nation of these factors that imply increased transport times rather than magma interacting with older crust). Increases in trace element ratios such as La/Yb (Figure 1c), which are sensitive to the pressure-temperature KARLSTROM ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 2 Geochemistry, Geophysics, Geosystems 10.1002/2014GC005355 conditions for garnet stability, suggest thickening of crust [Haschke et al., 2002; Lee et al., 2007; DeCelles et al., 2009]. Furthermore, in the Sierra Nevada, California, where shallowing of the slab is widely accepted to have driven migration of the arc front, xenolith data show that the Sierran arc root extended to depths of at least 90 km, approaching or even exceeding the average depth to the slab beneath modern arcs [Ducea and Saleeby, 1998; Saleeby, 2003]. This thick Sierran arc root apparently developed during the peak of arc magmatism, due to a com- bination of magmatic thickening and lithospheric shortening [Barth et al., 2012; Chin et al., 2012]. Thermobaromet- ric studies indicate that this thickening root impinged directly against a normally dipping slab [Chin et al., 2012]. None of the above observations require that arc front migration is caused by changing dip of the down- going slab. A number of mechanisms have been proposed for the transient flattening of slabs: subduction of oceanic plateaux [Saleeby, 2003], overthrusting or suction from deep continental roots [van Hunen et al., 2002], and evolving rheology of upper or downgoing plate [Billen and Hirth, 2007]. However, these mecha- nisms do not naturally explain the ubiquity and variability of arc front migrations or the unsteady magmatic output, and do not naturally explain the consistency between migration and geochemical data. Here we present a new model for arc front migration. Rather than relying on time-varying dip angle of the downgoing slab, we hypothesize that arc front migration occurs by the thickening of overlying crust and lithosphere due to melt extraction from the mantle wedge.